Coating thickness is a variable that plays an important role in product quality, process control, and cost control. Measurement of coating thickness can be done with many different instruments. The issues that determine what method is best for a given coating measurement include the type of coating, the substrate material, the thickness range of the coating, the size and shape of the part, and the cost of the equipment. Nondestructive thickness testing methods such as ultrasonic pulse-echo techniques, magnetic pull-off or electromagnetic induction based techniques using magnetic film gages, and eddy current based techniques are commonly used to measure the thickness of coatings in the industry.
Eddy current based techniques are typically used to measure the thickness of nonconductive coatings on nonmagnetic and conductive substrates. A coil of fine wire conducting an alternating current is used to set up an alternating magnetic field at the surface of the instrument's probe. When the probe is brought in contact with the surface of the coating, the alternating magnetic field will set up eddy currents on the surface of the conductive substrate. The coating acts as a spacer between the probe and the conductive substrate. As the distance between the probe and the conductive base metal increases, the eddy current field strength decreases because less of the probe's magnetic field can interact with the base metal. Electrical impedance, which is the total opposition that a circuit presents to alternating current, is used as a measure of the eddy current field strength. Electrical impedance, which is measured in ohms, includes three components—resistance, inductive reactance, and capacitive reactance. Since typical eddy current probes have very low capacitance, the capacitive reactance component can be ignored. The resistance and the inductive reactance (“reactance”) components of the impedance are out of phase, so the impedance is the vector sum of the resistance and reactance components. Typically, impedance measurements obtained from eddy current probes are displayed as an impedance plane plot, which is a graph with resistance on the x-axis and the reactance on the y-axis.
Specialized eddy current coating thickness gages that operate on this principle and display the thickness of a coating on an LCD screen are available to measure the thickness of nonconductive coatings on nonmagnetic conductive substrates. These gages use internal calibration curves to correlate the measured impedance magnitudes to a thickness value. If the phase information of the measured impedance is also recorded, thickness of conductive coating on ferromagnetic substrates may be obtained as well. A more versatile eddy current flaw detector may also be used to measure coating thickness using calibration specimens. The calibration specimens are used to establish calibration curves that plot the variation of the instruments response to coating thickness. The instruments response to a sample having an unknown coating thickness is then obtained using the calibration curve. Common practices of eddy current based coating thickness measurement are described in ASTM B244 standards for nonconductive coatings on nonmagnetic substrates. Another method utilizing an eddy current flaw detector for coating thickness measurement is described in U.S. Pat. No. 6,762,604 B2 issued to Le (“the '604 patent”). In the method of '604 patent, an eddy current monitoring system is used to measure the thickness of a coating on a semiconductor wafer using calibration curves. While the method of ASTM B244 and the '604 patent may be suitable to measure the thickness of a coating on a substrate having a constant conductivity, it may not be suitable to measure the thickness of a coating when the conductivity of the substrate changes due to the deposition process.
The disclosed method of thickness measurement is directed to overcoming one or more of the problems set forth above.